A radiographic image is an image of a spatial intensity distribution of a radiation ray, i.e., an image of a transmittance distribution of a radiation ray, which has transmitted through an object or a human body serving as an object of shooting. A parameter that causes a problem in a case of observing such radiographic image includes an existence of scattered radiation emitted by an object of shooting. In general, a radiographic image is generated using a radiation intensity at an arbitrary position of an image reception surface, which is obtained when an object of shooting lies between the radiation source and image reception surface, as an attenuation factor of the radiation ray on the straight line connecting the arbitrary position and the radiation source. In other words, because a radiation transmittance is obtained for the part of the object that includes the straight line, internal data of a local portion of the object can be obtained on a plane. This is called a direct radiation component.
However, a radiation ray that reaches the image reception surface includes, besides the direct radiation component, a scattered radiation component which is secondarily emitted by the object of shooting itself. Therefore, the radiation intensity is a sum of the direct radiation component and the scattered radiation component. Generally the radiation intensity is expressed by the number of energy particles reaching at random. Therefore, it is known that the radiation intensity obtained on the image reception surface naturally has a Poisson distribution, and that the dispersion (scatter value) is equal to the number of energy particles. Such dispersion causes random noise on the image reception surface. This is equivalent to an addition of scattered radiation that adds noise irrelevant to the shooting object data to the image data obtained on the image reception surface. Since the addition of scattered radiation is equivalent to an addition of random noise, contrast data of an extremely small object of shooting that is buried in the noise may completely be lost. In such case, it is extremely difficult to remove only the scattered radiation component from the image having the scattered radiation by calculation (image processing). As matters stand, it is considered almost impossible.
Since the outset of the X-ray discovery, the use of an anti-scatter grid is available as means for effectively removing the scattered radiation. FIG. 10 is an explanatory view on how to use the anti-scatter grid. Numeral 501 denotes an image reception surface; 502, an anti-scatter grid (hereinafter referred to as the grid); and 503, an object of shooting. The grid 502 is configured with a plurality of lead strips arranged toward the direction of an X-ray source 504 (X-ray focal point). While the direct radiation component from the X-ray focal point passes through the space between the strips and reaches the image reception surface as indicated by dashed lines 505, since the lead strips have a high X-ray blocking rate, the scattered radiation component having an orientation different from that of the direct radiation component is blocked by the strips as indicated by the arrow 506 and does not reach the image reception surface. By this method, a fair amount of the scattered radiation component can be removed. Although a part of the direct radiation component is sacrificed, it is possible to obtain a high-contrast image having little influence of the scattered radiation.
An adverse effect of the grid 502 is that a striped pattern (hereinafter referred to as gridlines), which is a shadow image of the strips, appears on the image reception surface 501. The gridlines on the image are sometimes a disturbance when the image is observed. An effective way of eliminating the gridlines is to move or oscillate the grid in the gridline traversing direction during an X-ray exposure. According to this method, the gridlines are blurred and only the shadow image of the shooting object appears on the image reception surface. However, this method requires a mechanism for moving the grid during an X-ray exposure, speed control for appropriately moving the grid, a countermeasure for oscillation, a countermeasure for noise generated by the moving mechanism, and so forth, thus requiring a high cost. In addition, since the timing of radiographing has to be adapted to the movement of the grid mechanism, there is a problem of less flexibility.
Meanwhile, since the image of gridlines is multiplied by the shadow image of the shooting object, it can be converted to addition data by logarithmic transformation. Therefore, if the image of shooting object having gridlines is acquired as a digital image, it is possible to remove the gridline data by image processing.
In general, the grid is constructed by arranging lead strips at extremely accurate intervals in one direction. The gridline's shadow image on an image has a spatially oscillating striped pattern having accurate spatial frequency peaks. Image processing methods for removing such gridline data include: a method disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 2001-189866 where a response of a particular spatial frequency area is reduced by using wavelet transformation; or a method disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 2002-330344 where a gridline component representing the characteristic of the gridlines is predicted and generated, then subtracted from the radiographic image.
In recent years, a so-called direct type X-ray sensor, in which incident X-ray particles directly capture free electrons generated in the semiconductor, is on its way to practical application, and the spatial frequency response (hereinafter referred to as the modulation transfer function (MTF)) of an X-ray image reception system (hereinafter referred to as the X-ray sensor) is improving. The spatial frequency response is improving also in the indirect type X-ray sensor, in which an X-ray intensity is once converted to low-energy fluorescence to be captured. According to the sampling theorem, all the components higher than the Nyquist frequency which is a half of the spatial sampling pitch are folded back to a frequency lower than the Nyquist frequency before they are observed. Therefore, it seems nonsense to improve the MTF more than necessary. However, it is better to improve the MTF as much as possible for a frequency lower than the Nyquist frequency. Therefore, the X-ray sensor's MTF is improved under the assumption that there is almost no component higher than the Nyquist frequency in an object of shooting.
FIG. 11A shows a shadow image of the gridlines. As illustrated, the light transmitting portion and blocking portion create periodical shadow images at accurate interval Tg. FIG. 11B is a schematic view of the spatial spectrum of FIG. 11A. Since the shadow image of the gridlines is a periodic function, FIG. 11B shows the 1/Tg line spectrum peak that is a fundamental wave, as well as the n-th (n=2, 3, . . . ) line spectrum peaks (n-th harmonics) of the fundamental wave. As the X-ray sensor's MTF is improved as mentioned above, the n-th line spectrum peaks are also transmitted without being reduced, and appear on the image. The conventional image processing technique of reducing the gridlines gives almost no consideration on removing the gridline component that corresponds to the n-th line spectra. The only thing that considers this matter is Japanese Patent Application Laid-Open (KOKAI) No. 2003-38481, which discloses a method of setting a frequency of gridlines in a way that the fundamental wave and second harmonic component of the gridlines have an almost equal frequency in the sampled image signal.